This invention relates to beam trawl type fishing nets, and more particularly, to beam trawl type fishing nets used on shrimp boats. It specifically relates to the method of handling the beam trawl on the fishing boat.
Shrimping today is done with a large variety of boats and net sizes. In general, the vast majority of shrimp are taken by trawling a net across the bottom of the ocean. Net size and the number of nets fished vary depending on the capabilities of the boat. The largest boats will trawl with 4-40 ft. nets. The nets are pulled with steel cable attached through outriggers extending from the sides of the boat. The nets consist of 2 sides a top and a bottom. These are attached together to form a net with a roughly rectangular front cross-section which reduces in size from front to back of the net. In the rear of the net a "bag" is attached. This bag receives all the catch from the net and can be closed at the top and pulled with a winch aboard the boat. The bottom of the bag can be opened thus releasing the catch on the boat. The main body of the net is left hanging over the side of the boat while the bag is recovered. Once emptied, the back of the bag is re-closed and the entire net is ready for fishing. The top front of the net is fitted with a head rope (or cork line) which can have floats attached to raise the top of the net higher. The bottom of the net is hung with chain so that it drags the bottom. "Tickler chains" are suspended several feet ahead of the net. These chains drag the bottom and kick up shrimp which are then caught by the net.
Two methods are currently used to keep the front of the net open to catch shrimp. In the currently most widely used method, devices called doors are attached to the outer edges of the front of the net. These doors are large flat devices which are rigged vertically in the water. When the boat goes forward, water pressure on the doors causes them to travel outward thus spreading the net. This is equivalent to dragging a funnel through the water. The doors drag over the bottom thus stirring up mud and debris which enters the net. To keep the net open the boat must travel approximately 2.8 mph. At a lower speed, the doors will either collapse toward the center of the net or roll over thus stopping the net from fishing. Care must be taken when setting the nets overboard since the doors will flip over if not properly placed in the water and against the tide (the tide keeps the net ballooned as the doors are lowered). Additionally, the doors are not effective on rough bottoms as they dig into dips etc. In certain areas where offshore drilling has taken place, a sediment called " blue mud" exists. When a rig with doors encounters this condition, either the doors dig into the mud and fill the net, or the entire rig can be lost from the boat.
Large amounts of cable are required as the rig needs to be pulled almost horizontally to function.
On large boats using multiple nets, a metal sled is placed between two nets and doors are placed on the outer edge of each net. In this configuration, a cable is attached to the sled and cables are attached to the doors. The three cables are brought together and pulled as one unit. The sled is a metal framework with a flat bottom much like a ski. The sled travels over the bottom and is kept upright by the pull of the doors on either side. On one side of a large boat one would find two doors, a sled and 2-40 ft. nets. In the water, since the nets are spread by water pressure, only 2/3 of maximum net opening exists. 2-40 ft. nets have a net fishing width of 52 ft. (26 ft. ea.).
Large horsepower boats are required to overcome the resistance of water on the doors. As the nets fill with catch, mud or debris, the pull of the net backwards pulls the doors inward thus reducing the net opening further. Increased speed (and horsepower) are used to overcome this. Eventually the nets will close completely unless brought up and emptied.
Problems are encountered when executing a turn with the nets in the trawling position. A turn must be made very large to keep the inside door on the net from rolling over and collapsing the net.
The life of the current door type trawl is very limited. The doors plow the bottom and need replacement every 6 to 12 months. If they are used on a rocky bottom, the doors can be destroyed at any time by striking a rock.
The popularity of this type of unit is due totally to the ease with which it is handled on the fishing boat. In general fishing boats, and in particular shrimp fishing boats, have large power winches located in the center of the boat. Outriggers which can be raised for docking or lowered for fishing extend up to fifty or more feet from each side of the boat. A large block is located at the end of each outrigger. Steel cable one half inch in diameter or larger runs up each outrigger and over the block at the end from the winch. The various cables attached to the fishing trawl are attached to the end of this cable. In the horizontal, or fishing, position, the block at the end of the outrigger is approximately fifteen feet above the water.
When a door trawl unit is raised, the cables attached to the trawl travel over the outrigger block and down to and around the winch. The doors come together and are raised completely clear of the water at the end of the outriggers. In this position, the doors are out of the water and out of the way. This process is repeated each time the net is emptied, which can be as frequently as once per hour.
An alternative to the doors on the outside edges of the nets has existed since at least the 1300's. It consists of a beam which spreads the net to its full width. This has been used in a limited manner in Alaskan waters (very rough bottom) for 40+ years. The net is spread vertically on the sides with "plumb staffs" which are 3 or 4 foot posts loosely attached at their midpoint to the ends of the beam. The sides of the net are attached to the plumb staffs and the top and bottom of the net are the same as in the normal trawl. This trawl is particularly effective on very steep slopes. A more modern version of the beam trawl consists of a metal beam supported between two metal sleds. The sleds spread the net vertically and support the beam a set distance from the bottom. The top of the net is laced to the beam and the bottom is free to drag the bottom slightly behind the top of the net. The sleds have curved bottoms to allow them to slide easily over the bottom.
Beam trawl units have none of the disadvantages of the door type trawls. The always remain open, they do not stir up the bottom and collect debris and they slide over soft areas. Beam trawl units require significantly less horsepower to operate and have proven to be more efficient fishers than door trawl units.
The most serious disadvantage of a beam trawl is the difficulty in safely and conveniently handling it on the boat. Large boats require beam trawls of fifty or more feet in length. The beam trawl must be pulled with three or more cables. As a minimum one cable attaches to the center of the beam and one cable attaches to each sled, or each end of the beam. The three cables are brought together and attached to the boats main cable in the same manner as the door trawl.
While another advantage of the beam trawl is the shorter cables required, they are still generally as long as the beam. A fifty foot beam trawl would have approximately fifty foot cables attached to it. When the trawl is raised by the boat winch, the attachment of the three cables cannot go over the block. If it is allowed to do so, the beam will be bent by the outer cables tightening at a faster rate than the center cable. Since the end of the outrigger is only fifteen feet above the water, a beam trawl of over fifteen or so feet cannot be raised clear of the water without raising the outrigger. In heavy seas, raising the outrigger each time the net is emptied is a dangerous and time consuming task. When the outrigger is raised, the beam swings in toward the boat. Even in calm weather, extra crew are required to guide the beam and insure that it does not strike the boat.
Storage of the beam trawl also presents a problem. Since the trawl cannot be raised against the outrigger, it must be stored along side of the boat. In most harbors, this will not work as boats are tied side by side.
For these reasons, the beam trawl has been limited in use to smaller boats fishing in calm waters.